There are many applications which require the reproduction or fabrication of one-of-a-kind parts of complex geometry. These applications include dental prostheses, in-the-ear hearing aid housings, molds or implants to replace damaged bones, etc. In such applications, the replacement parts are unique. Most of the applications require a highly precise fit of the replacement part.
Dental prostheses require precise replacement parts. Traditionally, the production of dental prostheses or restorations use lost-wax casting techniques. A dentist first removes all the decay from the tooth and ensures the integrity of the patient's remaining tooth structure. The dentist then makes an impression of the remaining tooth structure and the other teeth in the same dental arch. Usually, the dentist also makes an impression of the teeth in the opposing dental arch. The impressions are made using an elastomeric material. The elastomeric material is mixed, placed in a mouth-shaped tray for support, placed in the patient's mouth over the teeth and surrounding tissues, and held in place until it "sets." Setting times vary with the materials used, but generally range from 2 to 12 minutes per impression. When the elastomeric material sets, it has a consistency similar to that of modeling clay, but retains an elastomeric property so that it can slide past the crest of convexity of the teeth without being permanently distorted. When removed from the mouth, the elastomeric material creates a three dimensional negative of the teeth and their relative positions to each other. After the impressions are made, dental stone (plaster) is poured into the impressions and allowed to harden. The elastomeric material is then removed and discarded. Pins are set into the bottom of the arch containing the tooth to be restored. A base of plaster is poured over the pins. When the plaster has hardened, the area of plaster supporting the model of the prepared tooth is sectioned out. The pins and the holes in a second pour of the plaster serve as guides, permitting the cast of the prepared tooth (called a die) to be removed and reinserted into the plaster cast. Excess plaster around the base of the die is removed and the position of the margin of the restoration marked. A separating medium is applied to the die. A wax pattern is then prepared directly on the die. If jaw interrelationships have been recorded, the wax pattern is mounted on an articulator. The articulator is used as a device to represent the motion of the teeth to each other. After the wax pattern is completed, it is sprued and invested for casting. When the investment material has set, the investment and pattern are placed in an oven where the wax is burned away. The remaining cavity in the investment is then filled with molten metal or other casting material. The casting is quenched and recovered. The sprue is removed and the casting is finished. Finishing operations involve removing all remaining investment material and polishing the surface. Surface polish is done by hand using an array of burrs and stones of increasingly finer grit. Final finishing is done with rouge on a soft bristle brush or rag wheel. After final finishing, the restoration is ready for the patient. The restoration is placed on the tooth and the occlusal, functional, and proximal contacts are checked. The quality of the margins is also evaluated.
The clinical criteria for acceptance include:
1. Marginal fit: A dental explorer is passed from restoration to tooth and from tooth to restoration with the tip of the explorer held perpendicular to the surfaces of both. If the interface between the restoration and tooth can be detected, the restoration should not be permanently seated but instead should be remade. Depending on the location of the margin relative to the gum tissue, a gap of 20 to 100 microns can be detected.
2. Occlusal fit: A mark should be made when the teeth tap together with a piece of occlusal marking paper placed between the teeth. The patient should not detect excessive pressure on that tooth or report any discomfort. All other teeth which contact without the restoration in place must also contact with the restoration in place.
3. Interproximal fit: A piece of dental floss should "snap" as it passes between the restoration and the adjacent contact tooth.
4. Functional fit: As the teeth move through their normal excursions, the restoration should not contact prematurely.
If the restoration can be modified to pass the acceptance criteria, it is cemented in place. If the restoration cannot pass these acceptance criteria, a new impression is made and the process begins again.